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Bureau of Mines Information Circular/1985 



Agglomeration and Heap Leaching of Finely 
Ground Precious-Metal-Bearing Tailings 

By G. E. McClelland, D. L. Pool, A. H. Hunt, and J. A. Eisele 




UNITED STATES DEPARTMENT OF THE INTERIOR 



CD 
C 

3 
m 
> 

c 

9* 



75! 

*f/NES 75TH A^ 



Information Circular 9034 



Agglomeration and Heap Leaching of Finely 
Ground Precious-Metal-Bearing Tailings 

By G. E. McClelland, D. L Pool, A. H. Hunt, and J. A. Eisele 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



^ 

\^> 



MV 







Library of Congress Cataloging in Publication Data: 




Agglomeration and heap leaching of finely ground precious-metal-bear- 
ing tailings. 

(Information circular ; 9034) 

Supt. of Docs, no.: I 28.27:9034. 

1. Precious metals— Metal lurgy. 2. Leaching. 3. Tailings (Metal- 
lurgy). 4. Agglomeration. I. McClelland, G. E. II. Series: Information 
circular (United States. Bureau of Mines) ; 9034. 



-TUW&AM" [TN759] 622s [669'. 21 



85-600087 



CONTENTS 



Page 



Abstract. 1 

Introduction. 2 

Bench-scale investigation of process variables 2 

Equipment , procedures , and materials 2 

Results and discussion 3 

Effect of binder addition on flow rate 3 

Effect of water addition on flow rate 4 

Effect of curing time on flow rate 4 

Application to other tailings materials 5 

Evaluation of pilot-scale agglomerating equipment 6 

Equipment , procedures , and materials 6 

Results and discussion 7 

Commercial application of tailings agglomeration technology 8 

Gold agglomeration-heap leaching in central Nevada 8 

Silver agglomeration-heap leaching in southeastern California 9 

Conclusions 11 

References 11 

ILLUSTRATIONS 

1. Effect of binder addition on percolation rate 4 

2. Effect of moisture content on percolation rate 4 

3. Effect of curing period on percolation rate 4 

4. Operation of disk pelletizer for agglomerating tailings 6 

5. View of Goldfield agglomerating plant 8 

6. Inside view of drum agglomerator at Goldfield operating at 50 tons/h of 

dry feed 8 

7. Agglomerated feed being placed on heap by radial-arm stacker at Goldfield 9 

8. Agglomerated feed on heap at Goldfield 9 

9. View of agglomerating plant in southeastern California 10 

10. Agglomerated tailings on stockpile in southeastern California 10 



TABLES 

1. Summary of experimental results for two tailings samples 5 

2. Column percolation leaching tests 5 

3. Leaching results from agglomerates produced on three types of 

agglomerating equipment 7 







UNIT OF MEASURE ABBREVIATIONS USED 


IN THIS REPORT 


°c 




degree Celsius 


in 


inch 


ft 




foot 


kW'h 


kilowatt hour 


ft* 




cubic foot 


lb 


pound 


ft/s 




foot per second 


lb/h 


pound per hour 


gal 




gallon 


min 


minute 


gal/min 




gallon per minute 


mm 


millimeter 


gal/(h-ft 


2 ) 


gallon per hour 
per square foot 


oz 
oz/ton 


troy ounce 

troy ounce of precious metal 


gal/(min* 


ft2) 


gallon per minute 
per square foot 


pet 


per short ton of ore 
weight percent 


g 




gram 


rpm 


revolution per minute 


h 




hour 


s 


second 


hp 




horsepower 







AGGLOMERATION AND HEAP LEACHING OF FINELY GROUND 
PRECIOUS-METAL-BEARING TAILINGS 

By G. E. McClelland, ' D. L. Pool, 2 A. H. Hunt, 3 and J. A. Eisele 4 



ABSTRACT 

During the 1970' s, the Bureau of Mines investigated a particle agglom- 
eration technique for improving the flow of leaching solution through 
heaps of clayey or crushed, low-grade gold-silver ores. This technology 
has been adopted on a broad scale by the precious-metal-processing in- 
dustry. This report presents information on the application of agglom- 
eration technology to finely ground precious-metal-bearing tailings. 
Two commercial operations that have benefited from agglomeration tech- 
nology are discussed. The technology is cost effective because of de- 
creased leach times and improved precious metal recoveries. 



Supervisory metallurgist (now with Heinen-Lindstrom Consultants, Reno, NV). 

Research chemist. 
^Physical science technician. 
Supervisory chemical engineer. 

Reno Research Center, Bureau of Mines, Reno, NV. 



INTRODUCTION 



The Western United States has many 
tailings materials from former mining op- 
erations that contain significant pre- 
cious metal values. Most of these tail- 
ings resources are too low grade or too 
small to warrant the capital expenditure 
to construct a conventional agitated cya- 
nide leaching circuit. Heap-leach cyani- 
dation is a low-capital, low-operating- 
cost method to recover precious metals 
from low-grade ores and wastes ( 1-2) . ^ 
Heap leaching has been successfully ap- 
plied to many low-grade, oxidized, dis- 
seminated gold ores since the early 
1970's. The Bureau of Mines developed 
agglomeration technology to improve heap 
leaching of clayey ores, and it has been 
rapidly adopted by the precious metal in- 
dustry (3-9) • 

In contrast to ores that contain a 
small percentage of fines, most tailings 
materials are more than 50 pet minus 200 
mesh, with no coarse material. The fine 
particles and clays, if present, prevent 
uniform flow of leaching solution through 
the bedded material. In extreme cases, 
the migration of fines and the swelling 
of the clay constituents blind the heap 
and prohibit any solution flow through 
the heap. It was necessary to determine 



the best procedure for agglomerating 
finely ground tailings materials because 
they do not respond to agglomerating 
techniques in the same manner as ores. 

Several types of agglomerating equip- 
ment are used by precious metal producers 
for agglomerating crushed ores and 
wastes. Some types of equipment are com- 
mercially available and others are custom 
designed and constructed. The commer- 
cially available types are agglomerating 
drums, pelletizing disks, and pug mills. 
Equipment that was custom designed and 
constructed include drop-belt conveyors, 
reverse-belt conveyors, and vibrating 
"staircase" chutes. Drums, disks, and 
pug mills impart a rolling action to ef- 
fect agglomeration; the conveyors and vi- 
brating chutes impart a tumbling or 
bouncing action to effect agglomeration. 
Both types of equipment produce suitable 
agglomerates from crushed material, but 
they might not be suitable for agglomer- 
ating finely ground tailings materials. 
A bouncing action may be detrimental for 
fine particle agglomeration because there 
is no coarse material present to act as 
nuclei for agglomerate growth, and the 
bouncing action may cause the green pel- 
let to break soon after it is formed. 



BENCH-SCALE INVESTIGATION OF PROCESS VARIABLES 



EQUIPMENT, PROCEDURES, AND MATERIALS 

Investigations to determine the best 
agglomeration parameters for fine tail- 
ings were conducted using 50-lb charges 
of material. The agglomerated material 
was leached in Plexiglas^ thermoplastic 
polymer columns 5 ft high and 5.5 in ID. 
Four inches of washed gravel in a perfo- 
rated container were placed in the bottom 
of the column to prevent the tailing ma- 
terial from plugging the solution outlet. 
The 50-lb charge of material placed on 

-•Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 

Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



top of the gravel container gave a bed 
height of 4 to 5 ft. The pregnant cya- 
nide solution from the column was pumped 
through three glass columns, each of 
which contained 30 g of minus 6- plus 16- 
mesh, coconut-shell activated carbon to 
adsorb the dissolved gold and silver. 
The barren solution from the carbon col- 
umns was recycled to the top of the 
leaching column. Pregnant and barren so- 
lutions were analyzed periodically 
throughout the leaching cycle. 

Two mechanical variables were important 
for tailings agglomeration: the method 

of solution application and the method of 
mechanical tumbling during the agglomera- 
tion step. Both parameters are difficult 
to evaluate quantitatively, and the in- 
formation given is based on observation. 



Methods of mechanical tumbling are dis- 
cussed in the equipment evaluation sec- 
tion of this report. 

A 39-in, rotating-disk pelletizer was 
used to agglomerate 50-lb batches of feed 
for the leaching columns. The air-dried 
tailings charge was placed on the pellet- 
izer, and binder was mixed in while the 
pelletizer was rotating. A controlled 
amount of liquid was slowly added from a 
graduated cylinder while the disk was 
running. The agglomerated feed was re- 
moved from the disk, placed in the leach- 
ing column and cured for a specified time 
at ambient conditions before percolation 
leaching. The column was capped to mini- 
mize drying of the green pellets. Previ- 
ous work with ores had shown that if the 
pellets dried too rapidly, partial break- 
down of the agglomerates occurred on wet- 
ting. Stronger agglomerates were pro- 
duced if they remained moist during cur- 
ing. Leaching was started by pumping 
solution on top of the charge. A layer 
of coco matting covered the bedded mate- 
rial to prevent erosion and to distribute 
solution uniformly. 

Baseline percolation leaching experi- 
ments were conducted on each sample to 
simulate conventional heap leaching. The 
dry tailings were placed into the leach- 
ing column and an alkaline solution (pH 
10.5) containing 2 lb NaCN per ton of 
solution was pumped onto the charge. 

Calculated head analyses and precious 
metal recoveries were determined by the 
amount of precious metal adsorbed on the 
carbon, the amount remaining in the final 
pregnant solution, and the values remain- 
ing in the leached residue. All solid 
samples were analyzed by fire assay. So- 
lution samples were analyzed by atomic 
absorption spectrophotometry. Maximum 
percolation rate measurements were made 
after leaching was completed. The mate- 
rial in the column was flooded with bar- 
ren leaching solution, and the rate at 
which the solution drained from the col- 
umn was measured. The high flow rates 
obtained under flooded conditions would 
be impractical in actual heap leaching 
operations but demonstrate that very 
stable, porous agglomerates are produced 
and do not break down under exaggerated 
leaching condition. 



Two samples of precious-metal-bearing 
tailings were used in the investigation 
to determine the best process parameters. 
One sample was from a tailings pile in 
the Comstock District of Nevada. The 
tailings were 65 pet minus 200 mesh and 
contained an average of 0.04 oz Au and 
2.0 oz/ton Ag. The second sample of 
tailing was from an old flotation mill in 
southeastern California. The tailings 
were 90 pet minus 200 mesh and contained 
an average of 1.4 oz/ton Ag. Both 
samples were siliceous and amenable to 
cyanidation. Detailed bench-scale data 
are presented only for the Nevada tail- 
ings sample because agglomerating param- 
eters for both samples were the same ex- 
cept for curing time, which was longer 
for the California sample. After deter- 
mining the best process parameters, sev- 
eral other tailings samples, which repre- 
sented a cross section of types of mate- 
rials likely to be candidates for heap 
leaching, were agglomerated. 

RESULTS AND DISCUSSION 

Effect of Binder Addition on Flow Rate 

Preliminary investigations showed that 
Portland cement alone was not an adequate 
binder for agglomerating finely ground 
tailings unless about 50 lb binder per 
ton of dry feed was used. A combination 
of lime and Portland cement, in equal 
proportions, provided adequate agglomer- 
ate strength and permeability with less 
total binder additions. 

Fifty-pound charges of the Nevada tail- 
ings were mixed with the lime and cement 
in ratios ranging from 0:0 to 25:25 lb of 
each binder per ton of dry feed. The ma- 
terial was tumbled on the pelletizer 
while 22 pet water was added and tumbling 
continued until it was agglomerated. The 
agglomerated feed was placed into the 
leaching column, cured for 72 h, and 
leached. Figure 1 shows that increasing 
the amount of lime and cement up to 15 lb 
of each per ton of feed markedly improved 
percolation rates. The binder supplied 
sufficient protective alkalinity during 
leaching to maintain the leaching solu- 
tion pH at 11. This amount of binder was 
added during subsequent experiments. 



400,- 




400i- 



5:5 10:10 15:15 20:20 

LIME: CEMENT, lb/ton feed 



25:25 



FIGURE 1. - Effect of binder addition on per- 
colation rate. 




15.0 



17.5 



20.0 22.5 

MOISTURE, pet 



25.0 



27.5 



FIGURE 2. - Effect of moisture content on per« 
eolation rate. 



Effect of Water Addition on Flow Rate 

Experiments were performed on charges 
mixed with 15 lb lime and 15 lb cement 
per ton of feed and cured for 72 h. The 
amount of water added to the dry mixture 
ranged from 15 to 27.5 pet. The results 
are shown in figure 2. Solution flow 
rate through the agglomerated tailings 
increased with increasing moisture, at- 
tained a maximum of about 360 gal/(h»ft 2 ) 
at 22.5 pet and then rapidly decreased to 
about 50 gal/(h«ft 2 ) at 27.5 pet mois- 
ture. Figure 2 shows that the best mois- 
ture for agglomerating the tailings was 
22.5 pet; however, moistures from 20 to 
25 pet produced acceptable agglomerates. 

Atomizing solution spray did not 
achieve agglomeration. With the addition 
of moisture and tumbling, the tailings 
continued to wet but did not form nuclei 
to start agglomerating, and a wet slurry 
was produced. Coarser sprays of droplets 
provided nuclei for agglomerate growth. 
When a water droplet contacted the dry 
tailings, it was immediately coated with 
fine material and formed the nucleus nec- 
essary for growth. The nuclei grew into 
pellets by a layering process (10). This 
two-stage pellet-formation process was 
observed on the disk pelletizer. 

Effect of Curing Time on Flow Rate 

Fifty-pound charges were mixed with 15 
lb lime and 15 lb cement per ton of feed, 



400 r- 




_L 



J 



96 



120 



48 72 

CURING PERIOD, h 

FIGURE 3. - Effect of curing period on perco- 
lation rate. 

wetted with 22 pet water, and agglomer- 
ated. The agglomerated charges were 
cured for 0, 2.5, 5, 7.5, 10, 24, 48, 72, 
96, and 120 h in capped leaching columns. 
Flow rates are presented in figure 3. 
For the Nevada sample, the data show that 
the duration of curing was important up 
to 24 h, after that time, no significant 
improvement in flow rate occurred. 

A summary of the best conditions deter- 
mined experimentally for the Nevada tail- 
ings and the California silver tailings 
is shown in table 1. Agglomeration pre- 
treatment increased the percolation rate 
from zero to more than 300 gal/(h»ft 2 ) 
for each sample. The precious metal 



extractions by simulated heap leaching 
were within 5 pet of those obtained by 
agitated-bottle leaching tests of the two 
samples . 

TABLE 1. - Summary of experimental 
results from two tailings samples 





Sample 




Nevada 


Cali- 
fornia 


Best pretreatment 






conditions : 






Lime: cement 






lb/ton feed. . 


15:15 


15:15 




22.5 
24 


18.0 




72 


Percolation rate, 1 






gal/(h-ft 2 ): 






Baseline, no pre- 














With best pre- 






360 

85.7 


431 


Gold extraction. . .pet. . 


NAp 


Silver extraction 






pet. . 


61.4 


74.2 



N'Ap Not applicable. 
'Percolation rate measurements were 
taken under flooded column conditions. 

Application to Other Tailings Materials 

Agglomeration process parameters were 
tested on the following other tailings 
samples : 

• A gold-bearing tailings material 
from a former cyanide mill in Montana. 



The tailings were siliceous and contained 
granular sandy material. The feed was 50 
pet minus 200 mesh and the coarser parti- 
cles were minus 20 mesh. The sample con- 
tained 0.05 oz/ton Au and trace amounts 
of silver. The natural pH of the mate- 
rial in a 50-pct slurry was 4.6. 

• A gold-bearing tailings material 
from southern Nevada that contained 0.13 
oz/ton Au and 0.14 oz/ton Ag (southern 
Nevada 1). The tailings were 80 pet 
minus 200 mesh and had been previously 
cyanided. The natural pH of the material 
in a 50-pct slurry was 7.9. 

• A tailings resource from southern 
Nevada that contained 0.03 oz/ton Au and 
1.0 oz/ton Ag (southern Nevada 2). The 
tailings were 90 pet minus 200 mesh and 
contained significant amounts of clayey 
material. The natural pH of the material 
in a 50-pct slurry was 6.3. 

Results from the column percolation 
leaching experiments are shown in table 
2. Precious metal extractions from 
agglomeration-heap leaching were within 5 
pet of those obtained by agitated cyani- 
dation of the same size feed material. 
Agglomeration pretreatment markedly im- 
proved percolation rate and gold recov- 
ery, and decreased the leaching time com- 
pared to conventional heap leaching. The 
bench-scale results for the tailings sam- 
ples demonstrate that agglomeration pre- 
treatment provides a means of processing 
precious-metal-bearing tailings that 
otherwise might not be exploited. 



TABLE 2. - Column percolation leaching tests 



Montana 


Nevada 1 


Nevada 2 


Base- 


Agglom- 


Base- 


Agglom- 


Base- 


Agglom- 


line 


erated 


line 


erated 


line 


erated 


0.05 


0.05 


0.12 


0.13 


0.03 


0.03 


NAp 


NAp 


NAp 


NAp 


1.0 


1.1 





20:20 





10:10 





15:15 





22.0 





15.7 





18.0 


0.08 


119 


0.009 


510 





533 


4 


2 


27 


3 


NAp 


6 


40.0 


80.0 


16.0 


80.0 


NAp 


76.7 


NAp 


NAp 


NAp 


NAp 


NAp 


69.0 



Calculated head, oz/ton: 

Gold 

Silver 

Binder, lime:cement lb/ton feed.. 

Moisture pet. . 

Percolation gal/(h»f t 2 ) . . 

Leaching period days. . 

Recovery, pet: 

Gold 

Silver 



NAp Not applicable. 

NOTE. — Curing period for all samples was 72 h. The solution contained 2 lb NaCN 
per ton of solution. Flow rate measurements taken under flooded conditions. 



EVALUATION OF PILOT-SCALE AGGLOMERATING EQUIPMENT 



EQUIPMENT, PROCEDURES, AND MATERIALS 

Several types of equipment were eval- 
uated to determine the best type for 
tailings agglomeration. They were (1) 
reverse belt agglomerator (RBA), (2) drum 
agglomerator, and (3) disk pelletizer. 
Each piece of equipment was designed to 
agglomerate a minimum of 500 lb/h of dry 
tailings. The drum and the RBA were 
fabricated locally; the disk was pur- 
chased from an agglomeration equipment 
manufacturer. 

A 3-ton sample of the tailings from the 
Comstock District of Nevada on which the 
initial tailings agglomeration research 
was done was used in the equipment evalu- 
ation. The sample was air dried, blend- 
ed, and placed into sealed containers. 
Five-hundred-pound charges of tailings 
were premixed with 15 lb lime and 15 lb 
cement per ton of feed in a double cone 
dryer blender. Premixing of binder and 
tailings eliminated proper binder mixing 
as a variable in evaluating agglomerating 
equipment. 

The RBA was constructed from a 12-ft by 
10-in flat conveyor with variable belt 
speed and belt angle. To insure an ad- 
equate bed depth of material being ag- 
glomerated, the working width of the belt 
was decreased to 4 in by constructing 
sheet metal walls on the surface of the 
belt. Deflector plates attached to the 
sheet metal walls directed bouncing ag- 
glomerates back onto the belt surface. 
The binder and tailings charge was fed 
from a 5-ft 3 hopper onto the belt by a 
vibrating feeder at a point 28 in from 
the top of the working length of the 
belt. The working length of the belt was 
6 ft. Moisture was added along the belt 
through a drip irrigation system. At the 
uppermost moisture addition point, ap- 
proximately 8 in above the dry feed addi- 
tion point, 20 pet of the moisture was 
added to the feed on the belt. Two other 
moisture addition points were below the 
dry feed addition point. The RBA oper- 
ated best at an angle of 50°, a belt 
speed of 4.1 ft/s, and a feed rate of 450 
lb/h. The estimated retention time of 
feed on the belt was 10 s. 



The disk pelletizer was 3 ft in diame- 
ter and 8 in deep; pan depth, angle, and 
rotational speed were variable. Figure 4 
shows how the disk was operated for best 
pellet formation. Moisture was applied 
through nozzles that delivered droplets 
at a rate controlled by a metering pump. 
The disk operated best at a pan angle of 
56°, at 45.5 rpm, and at a feed rate of 
2,720 lb/h of dry feed. Agglomerate re- 
tention time was 2 min. 

The drum agglomerator was 12.6 in ID, 
37.5 in long, and rotated by a 1/3-hp 
belt drive motor. The angle and speed of 
the drum were variable to control reten- 
tion time. The drum was lined with 
loosely fitted conveyor belt material to 
minimize moist feed buildup on the drum 
walls. When the drum reached the top of 
its rotation, the lining sagged and dis- 
lodged the adhering feed. A spray bar 
situated lengthwise in the drum delivered 
amounts of water controlled by a metering 
pump through three nozzles. The nozzles 
delivered moisture droplets in a fan pat- 
tern which covered almost two-thirds the 
length of the drum. The drum operated 
best at an angle of 1.5°, at 37.5 rpm, 
and at a feed rate of 2,800 lb/h of dry 




Feed input 



KEY 

51 Nucleating spray location 

52 Agglomerate building spray location 

FIGURE 4. - Operation of disk pelletizer for 
agglomerating tailings. 



feed. Agglomerate retention time was 42 

s . 

RESULTS AND DISCUSSION 

Fifty-pound charges of agglomerates 
produced under the best conditions for 
each type of equipment were placed in 
capped bench-scale leaching columns, 
cured for 72 h, and leached. The leach- 
ing tests determined agglomerate strength 
and stability under leaching conditions 
and precious metal recovery. A solution 
containing 2 lb NaCN per ton was pumped 
through the agglomerates at a rate of 5 
gal/(h*ft 2 ). Results from leaching are 
shown in table 3. 

TABLE 3. -Leaching results from 
agglomerates produced on three 
types of agglomerating equipment 





Type 


of equi 


pment 




RBA 


Disk 


Drum 


Calculated head, 








oz/ton: 








Gold 


0.035 


0.037 


0.039 


Silver 


1.26 
5.5 


1.26 
None 


1.27 


Bed slump in.. 


None 


Fines migration 










Yes 


No 


No 


Percolation rate 








gal/(h-ft 2 ).. 


7.0 


604 


435 


Leaching period 








days. . 


7 


3 


3 


Recovery, pet: 








Gold 


73.7 
53.2 


85.3 
61.1 


86.0 




51. 1 






NOTE: — Flow rate 


measu 


rements 


were 


taken under flooded c 


onditio 


ns. 





The agglomerates produced on the RBA 
were of various sizes and contained some 
unagglomerated fines. Observations dur- 
ing column percolation leaching of the 
agglomerates showed severe slumping and 
fines migration and indicated that the 
agglomerates did not have sufficient 
green strength to withstand percolation 
leaching. The lack of competency was at- 
tributed to insufficient retention time 
of agglomerates on the belt. If the feed 
was allowed to remain on the belt for 
sufficient time to form proper pellets, 



the green pellets broke apart as they 
bounced down the belt. If the retention 
time of feed on the belt was decreased to 
overcome agglomerate bouncing, there was 
insufficient time for proper pellet for- 
mation. The RBA could not tolerate fluc- 
tuation in feed and moisture addition 
rates and the bed depth was difficult to 
control. The RBA did not produce accept- 
able agglomerates. 

The green pellets formed on the disk 
pelletizer under the best operating 
conditions were uniform in size at ap- 
proximately 1/2 in diam. No slumping or 
fines migration was observed during 
leaching. The agglomerates were compe- 
tent, permeable, and had sufficient 
strength to withstand percolation leach- 
ing. The best agglomerating conditions 
for the disk were easy to maintain con- 
stant. However, a change in one param- 
eter affected the other parameters. For 
example, if the pan angle was changed, 
agglomerate retention time changed and 
affected agglomerate size, feed rate, and 
moisture addition rate. Changes in disk 
operation can be observed immediately by 
the operator. 

The drum agglomerates fine particles in 
the same manner as the disk, but the ag- 
glomerates vary in size. The agglomer- 
ates with a range of sizes could be 
stacked with a steeper angle of repose 
than could the uniform-sized agglomerates 
produced by the disk. The green pellets 
produced by the drum agglomerator operat- 
ing under the best conditions varied in 
size from 3/4 in diam to approximately 10 
mesh. No slump or fines migration was 
observed during leaching. The agglomer- 
ates were competent, permeable, and 
strong enough to withstand percolation 
leaching. It was difficult to maintain 
steady-state conditions in the drum be- 
cause of feed surging, which caused non- 
uniform moistening of the agglomerates. 
The situation could be improved by a 
better designed feeder system; however, 
the drum agglomerator will tend to have a 
pulsed output. Changes in operating con- 
ditions were more difficult to monitor 
than with the disk. Since the operator 
can only observe the agglomerates as they 
discharge from the drum, a waiting period 



equal to the feed retention time occurs 
before the effect of a change can be 
determined. 

The disk pelletizer and drum agglomer- 
ator are the types of equipment most 
suitable for tailings agglomeration. The 
disk is favored because of its ease of 
operation. The disk is also slightly 
favored in capital cost to tonnage 



throughput ratio based on capital cost 
estimates from three agglomeration equip- 
ment manufacturers. The drum is favored 
because the agglomerates produced are 
varied in size, which improves their 
stacking characteristics on a heap. Se- 
lection of agglomerating equipment should 
be based on the specific needs of each 
agglomeration operation. 



COMMERCIAL APPLICATION OF TAILINGS AGGLOMERATION TECHNOLOGY 



Tailings agglomeration-heap leaching is 
being used at least a minimum of two com- 
mercial operations. The operators per- 
formed bench-scale conventional heap 
leaching tests on their materials. Re- 
sults showed that without agglomeration, 
they could not be heap leached because of 
the extremely poor percolation character- 
istics of the materials. 



lime-cement slurry was applied through 
the spray system to add binder and bring 
the final moisture content of the agglom- 
erates to between 12 and 14 pet. The 
total binder addition was 50 lb lime and 
10 lb cement per ton of dry feed. The 
large lime addition was required to ad- 
just the pH of the tailings from 1.7 to 
10.5. A 12-in weir on the inside of the 



GOLD AGGLOMERATION-HEAP LEACHING IN 
CENTRAL NEVADA 

A tailings material from the central 
Nevada Goldfield District was processed 
by agglomeration pretreatment and heap 
leaching cyanidation (fig. 5). The tail- 
ings resulted from a cyanide milling op- 
eration that was active just after the 
turn of the century. The original ore 
was high in sulfides and gold recoveries 
were low. The tailings oxidized for ap- 
proximately 70 yr by natural weathering 
and the residual sulfides oxidized to 
soluble sulfate. The natural pH of a 50 
pet solids slurry was 1.7 because of the 
soluble sulfates. The tailings were 65 
pet minus 200 mesh and contained 0.08 
oz/ton Au. The maximum gold recovery by 
agitated cyanidation was 83 pet. 

The tailings were moved to the agglom- 
erating plant by a front-end loader and 
dumped into a hopper. The tailings were 
conveyed to a 8-1/2- by 22-ft drum ag- 
glomerator, which was a modified asphalt 
kiln. The drum rotated at 10.5 rpm, had 
a slope to the discharge end of 4°, and 
was lined with loosely fitting conveyor 
belt material. A spray bar was situated 
lengthwise in the drum and delivered a 
fan droplet spray that covered three- 
fourths of the length of the drum. A 




FIGURE 5. - View of Goldfield agglomerating 
plant. 




FIGURE 6. - Inside view of drum agglomerator at 
Goldfield operating at 50 tons/h of dry feed. 



drum was 4 ft from the discharge end to 
increase feed retention time and to pre- 
vent discharge surging (fig. 6). The ag- 
glomerates discharged from the drum to a 
transfer point feeding a radial arm 
stacker. The green agglomerates were 
gently placed on the heap by the stacker 
and cured during heap building. 

The leaching pad was constructed by 
compacting barren tailings in 6-in layers 
and covering them with a 20-mil PVC lin- 
er. The heaps were built by adjusting 
the radial-arm stacker to its lowest 
angle, sweeping across the width of the 
pad, raising the stacker discharge end 1 
ft, and sweeping the opposite direction 
across the width of the pad. This proce- 
dure continued until the heap was 16 ft 
high. The stacker remained at 16 ft, and 
new agglomerates were added to the heap 
by sweeping the stacker across the width 
of the pad and allowing the agglomerates 
to cascade down the heap (fig. 7). Heaps 
built in this manner avoided compacting 
the agglomerates. The agglomerating 
equipment and the stacker were moved as a 
unit by a dozer before a new row of ag- 
glomerates was added to the leaching pad. 
A width of PVC liner was rolled out the 
width of the pad and welded as necessary 
to continue heap building. The agglomer- 
ated tailings are shown in figure 8. The 
agglomerates cured for several days while 
the 6,400-ton heap was built. The heap 




was leached by spraying a cyanide solu- 
tion containing 2.0 lb NaCN per ton of 
solution on it at a rate of 0.003 gal/ 
(min*ft 2 ). The pregnant solution that 
drained from the sloped leaching pad col- 
lected in lined ditches and flowed by 
gravity to a pregnant solution pond. 
Gold was recovered from the pregnant so- 
lution by carbon adsorption-desorption- 
electrowinning. 

Gold recovery by agglomeration-heap 
leaching was 76 pet. Cyanide consumption 
was 0.7 lb NaCN per ton of tailings. The 
leaching-washing cycle was about 24 days. 

SILVER AGGLOMERATION-HEAP LEACHING IN 
SOUTHEASTERN CALIFORNIA 

A tailings pile from a former flotation 
operation in southeastern < California was 
processed by agglomeration-heap leaching 
to recover the contained silver values 
(11). The tailings were 90 pet minus 200 
mesh and contained an average of 1.4 oz/ 
ton Ag. The operators of the property 
conducted both bench- and pilot-scale ex- 
periments to evaluate agglomeration-heap 
leaching for recovering silver from the 
tailings. Bench-scale results showed 
that 63 pet of the silver was recovered 
by agitated cyanidation and 72 pet of the 
silver was recovered by agglomeration- 
heap leaching. 




FIGURE 7. - Agglomerated feed being placed on 
heap by radio Norm stacker at Go I dfi eld. 



FIGURE 8. - Agglomerated feed on heap at 
Goldfield. 



10 



The plant was designed to agglomerate 
and heap leach 100 tons per day of dry 
tailings. The tailings were mined and 
crushed to break up the large chunks of 
clayey material. The crusher discharge 
was conveyed to two mixers where 35 lb 
Portland cement per ton of dry feed was 
added as a binder. The binder and feed 
mixture was conveyed to a 10- by 30-ft 
drum agglomerator. A scraper, which ro- 
tated in the opposite direction to the 
drum, was situated along the length of 
the drum to prevent agglomerate buildup. 
Water was applied as a coarse spray 
through a spray bar lengthwise in the 
drum. The water was added at the bottom 
of the drum and mixed with the tailings 
to bring the final moisture content to 12 
to 15 pet. The agglomerates discharged 
the drum to a transfer point feeding a 
radial-arm stacker. The green agglomer- 
ates were placed on a stockpile by the 
stacker and cured 4 days before being 
transported to the leaching pad. A view 
of the operation is shown in figure 9. 
The agglomerated tailings are shown in 
the stockpile in figure 10. 



The leaching pad was constructed from 
moistened clayey silt material compacted 
in three layers. The middle layer was 
mixed with bentonite to insure impermea- 
bility of the leaching pad. A cushioning 
layer of sand was placed on the prepared 
pad to minimize pad degradation by equip- 
ment during heap construction. The heaps 
were built to a height of 13 ft by a 
front-end loader to avoid driving on the 
agglomerates. 

The heaps were sprayed with a solution 
containing 2.0 lb NaCN per ton of solu- 
tion and at a rate of 0.01 gal/(min»f t 2 ). 
The pregnant solution collected in lined 
ditches and drained by gravity to a preg- 
nant solution pond. The pregnant solu- 
tion contained an average of 1.0 oz Ag 
per ton of solution and was pumped to the 
precipitation circuit at a rate of 150 
gal/min. The solution was clarified, de- 
aerated, and contacted with zinc dust to 
recover the silver. Silver recovery was 
73 pet after a 25-day leaching and 40-day 
washing cycle. 





FIGURE 9. - View of agglomerating plant in 
southeastern California. 




FIGURE 10. - Agglomerated tailings on stock- 
pile in southeastern California. 



CONCLUSIONS 



11 



Agglomeration pretreatment makes pos- 
sible heap leaching of very fine materi- 
al, such as tailings. A mixture of lime 
and cement was the most suitable binder. 
Moisture addition was critical. If too 
much or too little was added, the materi- 
al did not agglomerate. A curing time of 
24 h was required. The mechanical method 
of tumbling the moistened mixture was 



important in agglomeration. Any equip- 
ment that imparted a bouncing action pre- 
vented good pellet formation. When these 
criteria were met, strong, durable, and 
porous pellets that could readily be heap 
leached were produced and the processing 
of low-grade and/or small resources that 
are in a finely divided state is 
possible. 



REFERENCES 



1. Heinen, H. J., D. G. Peterson, and 
R. E. Lindstrom. Processing Gold Ores 
Using Heap Leach-Carbon Adsorption Meth- 
ods. BuMines IC 8770, 1978, 21 pp. 

2. Potter, G. M. Recovering Gold From 
Stripping Waste and Ore by Percolation 
Cyanide Leaching. BuMines TPR 20, 1969, 
5 pp. 

3. Heinen, H. J., G. E. McClelland, 
and R. E. Lindstrom. Enhancing Percola- 
tion Rates in Heap Leaching of Gold- 
Silver Ores. BuMines RI 8388, 1979, 20 
pp. 

4. McClelland, G. E., and J. A. 
Eisele. Improvements in Heap Leaching To 
Recover Silver and Gold From Low-Grade 
Resources. BuMines RI 8612, 1982, 26 
pp. 

5. McClelland, G. E. , D. L. Pool, and 
J. A. Eisele. Agglomeration-Heap Leach- 
ing Operations in the Precious Metals In- 
dustry. BuMines IC 8945, 1983, 16 pp. 

6. Lewis, A. Producing Gold for 
$160/Tr0z in Victor, Colorado. Eng. and 
Min. J., v. 183, No. 10, Oct. 1982, pp. 
102-105. 



7. Steele, G. L. Candelaria: Famous 
Silver Producer. Min. Eng., v. 33, No. 
6, June 1981, pp. 658-660. 

8. Engineering and Mining Journal. 
This Month in Mining. Alligator Ridge 
Uses Heap Leaching To Produce Gold Bul- 
lion Bars. V. 182, No. 8, Aug. 1981, pp. 
35-37. 

9. . Pelletizing Aids Tombstone 

Leaching Operation. V. 182, No. 1, Jan. 
1981, pp. 94-95. 

10. Kapur, P. C. Role of Similarity 
Size Spectra in Balling and Granulation 
of Coarse, Liquid Deficient Powders. Ch. 
10 in Agglomeration '77, ed. by K. V. S. 
Sastry (Proc. 2d Int. Symp. on Agglomera- 
tion, Atlanta, GA, Mar. 6-10, 1977). 
AIME, 1977, pp. 156-175. 

11. Milligan, D. A., and P. R. Engel- 
hardt. Agglomerated Heap Leaching Ana- 
conda's Darwin Silver Recovery Project. 
Pres. at Soc. Min. Eng. AIME Fall Meeting 
and Exhibit, Salt Lake City, UT, Oct. 19- 
21, 1983. Soc. Min. Eng. AIME preprint 
83-430, 9 pp. 



i-U.S. CPO: 1985-305-019/20.084 



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